Magnetic Quantum Tunneling in a Mn12 Single-Molecule Magnet Measured With High Frequency Electron Paramagnetic Resonance

Abstract

The low temperature spin dynamics of the single-molecule magnet [Mn 12 O 12 (CH 3 COOH) 16 (H 2 O) 4 ]⋅ 2CH 3 COOH·4H 2 O, hereafter Mn 12 Ac, were studied using High Frequency Electron Paramagnetic Resonance (HFEPR) in order to demonstrate magnetic quantum tunneling between resonant spin projection states. We prepare the spins such that they populate only one side of the axial potential energy barrier. Using a magnetic field we cause tunneling between resonant energy states. We then use HFEPR to monitor the populations on each side of the potential energy barrier. We can estimate the tunneling relaxation time between spin projection states by plotting the area of an EPR peak as a function of wait time at a resonance field. This technique provides an alternative method to magnetometry experiments for measuring spin relaxation dynamics.

Frequency-domain magnetic-resonance spectroscopic investigations of the magnetization dynamics inMn12Acsingle crystals

“…It should be noted that in ESR measurements, recently published by Hill et al, similar line shifts ͑by up to 0.28 T͒ and line shape changes were observed. 33 Similarly to the measurements presented in this manuscript, the origin of these shifts is presumably also the combined effect of internal dipolar fields and the influence of off-diagonal elements in the magnetic-permeability tensor. The shifts are too large to be explained by internal dipolar fields alone ͑see below͒.…”

“…31,32 Recently, a preliminary study of the magnetization relaxation in Mn 12 Ac by ESR was published. 33 However, for the study of magnetization relaxation and quantum tunneling of the magnetization, it is convenient to measure the magnetic-resonance spectra in the frequency domain rather than the field domain because the magnetization relaxation rate is field dependent, but not frequency dependent. In recent years, we have extensively used the technique of frequency-domain magnetic-resonance spectroscopy ͑FD-MRS͒ for the study of single-molecule magnets.…”

Landau-Zener problem with waiting at the minimum gap and related quench dynamics of a many-body system

“…In Ref. 23, the relaxation dynamics with waiting at a resonance was studied for single molecular magnets (SMM) called Mn 12 Ac, where each molecule has total spin S = 10. As the magnetic field in theẑ-direction is varied from a large negative to a large positive value, S z changes from 10 to −10.…”

“…In Ref. [23], the magnetic field is swept to a resonance value, starting from a field of −6 T , where it is held for different waiting times causing tunneling of the spins; eventually the field is brought back to its initial value. The number of molecules which have tunneled through the barrier, measured using electron paramagnetic resonance techniques, shows nearly a stretched exponential decay with t w , where the decay constant is governed by the relaxation time of the system.…”

“…We study the possible correction to the KZ power law scaling n ∼ 1/τ dν/(νz+1) , where d is the spatial dimension and ν and z are the correlation length and dynamical exponents associated with the QCP [10] across which the system is swept. To the best of our knowledge, these questions have not been addressed before from a theoretical point of view, although experimental results with waiting for single molecular magnets Mn 12 Ac are available [23]. The possibility of experimental realizations of quenching dynamics with waiting in optical lattices serves as another motivation.…”